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1. Graphical substrate
At present, LED epitaxy generally uses patterned substrate (PSS). PSS is currently divided into micro-scale PSS and nano-scale nPSS. Micro-scale PSS has various shape patterns. The height of the pattern is generally 1.1~1.6μm, and the diameter of the circle is 2.5~3μm. About 4μm, using light micro-projection and plasma dry etching technology, generally can improve the light efficiency by 30~40%. The nPSS generally adopts nanoimprint technology, and the pattern size is about 260 nm, and the period is about 460 nm, which generally improves the light efficiency by about 70%.
(1) nPSS substrate
The nano-template and substrate parallelism are demanding, nPSS advantages: LEDs have higher luminous efficiency, better uniformity and lower cost. For example, a hexagonal array with a period of 450 nm is obtained by nanoimprint lithography on a sapphire substrate, so that the output power of the green LED is three times that of the original.
(2) Nanopillar PSS
The new technology of the British company Seren uses a unique nanolithography technology on the sapphire substrate to form a nano-column on the surface. The epitaxial growth on the substrate can relieve the stress by 85%, thereby greatly reducing defects and improving the brightness of the light. Up to 80~120%, the industrialization level of LED light efficiency reaches 200lm/w, and the Droop effect is improved, and the attenuation is reduced by about 30%.
Summary: PSS can greatly improve the LED luminous efficiency, especially the nano-scale nPSS can greatly improve the LED luminous efficiency. PSS is the development trend of the current LED core technology. There are different views on PSS in terms of cost reduction.
2, homogenous substrate
The homogenous substrate is made of GaN as the substrate. There are various methods for growing the GaN substrate. Generally, HVPE (Hydride Vapor Phase Epitaxy) or sodium flow method is used. The GaN substrate is produced to solve the residual stress and surface roughness problem. The bottom thickness is about 400~500μm, and it can be industrialized. Advantages of GaN substrate: low dislocation density (105~106/cm2), internal quantum efficiency of over 80%, short growth time of about 2 hours, saving a lot of raw materials, and greatly reducing costs.
(1) Realize high brightness LED
Toyota Synthetic uses a c-plane GaN substrate to grow LED chips with an area of ​​1mm2, which can achieve 400lm luminous flux.
(2) HVPE growth GaN substrate industrialization
Mitsubishi Chemical, Sumitomo Electric, Hitachi Cable and other companies use HVPE to grow GaN substrates with a thickness of about 450μm and dislocation density (106~107pcs/cm2). Mitsubishi Chemical recently announced the availability of 6′′ GaN substrates, and plans to 2015. The cost will be reduced to one tenth of the current. Dongguan Zhongrong (PKU) can mass produce GaN substrates.
(3) Improve internal quantum efficiency
Japan Insulators uses a sodium flow method to grow GaN substrates with low defect density and internal quantum efficiency of 90%. At 200 mA, its luminous efficiency reaches 200 lm/w, which can provide 4′′ GaN substrates and is accelerating the development of low defects. "Substrate.
(4) Large size GaN substrate
Sumitomo Electric and Soitec have jointly developed 4" and 6" GaN substrates, using crystal garden manufacturing technology and intelligent lift-off layer transfer technology to produce ultra-thin high-quality GaN substrates with low defect density and announced the availability of GaN substrates.
(5) LiGaO2 substrate
South China University of Technology research and development on the LiGaO2 substrate by laser molecular beam epitaxy growth of non-polar GaN substrate, thickness 2μm, as a composite substrate growth GaN chip, required to achieve a dislocation density of 1 × 106 / cm2, internal quantum efficiency of 85% The conversion efficiency is 65%.
(6) Award-winning products
The US company Soraa used Nakamura Shuji's GaN-on-GaN technology to make LED replacement lamps, which was named "one of the most important achievements in semiconductor materials science in the past 30 years" by SVIPLA. Its LED crystal integrity has increased by more than 1,000 times, making it possible to use one LED device per lamp.
Summary: GaN-on-GaN homogeneous substrate growth LED, its defect density (105 ~ 106 / cm2), can greatly improve the LED luminous efficiency, and Droop is not obvious when increasing the current density, so that the general lighting is adopted The single-chip LED light source takes the LED core technology to a new level. Summary with Nakamura Shuji: We believe that with GaN-on-GaN LED, we have really written a new chapter in LED technology, namely LED version 2.0.
3, non-polar, semi-polar substrate
The sapphire (Al2O3) crystal plane has a polar C surface, a semipolar M surface, an R surface, and a non-polar A surface. With non-polar or semi-polar substrates, growth is difficult and the defect density can be greatly reduced. LEDs grown on non-polar substrates can be used as backlights for displays, TVs, mobile phones, etc., without orientation, no external diffusers, and can be used for recording, laser, solar panels.
(1) Non-polar, semi-polar sapphire substrate
The United States Salon Optoelectronics uses LEDs grown on non-polar sapphire to significantly reduce the defect density. The light conversion efficiency of the epitaxial wafer can be increased by 7 times, and the brightness is greatly improved to effectively improve the dollar/lumen value.
(2) "nPola" LED
Seoul Semiconductor uses a non-polar GaN substrate to grow LEDs called "nPola" LEDs, which achieve 500 lm of luminous flux on a 1 mm2 chip. Seoul Semiconductor CEO Li Yixun said: The brightness of the same surface is greatly improved by 5 times, and in the future it can be increased by more than 10 times. The ultimate goal of the light source.
(3) Non-polar GaN substrate
Mitsubishi Chemical uses a non-polar GaN substrate to grow blue LEDs with a defect density of at least 1 × 104 / cm 2 . The goal is to achieve a luminous flux of up to 1000 lm in a 1 mm2 chip.
(4) Industrialization of non-polar, semi-polar GaN substrates
Sumitomo Corporation has announced the development of semi-polar, non-polar GaN substrate materials that provide semi-polar, non-polar substrates for white LEDs.
(5) UV LEDs use non-polar substrates
Seoul Semiconductor uses a non-polar GaN substrate to develop UV LEDs and combines with R, G, and B phosphors to achieve high color rendering index white light illumination and a wide range of color rendering backlights.
Summary: The core technology of growing LEDs with semi-polar, non-polar sapphire and GaN substrates has made breakthroughs. It is possible to achieve 1000 lm luminous flux on a 1 mm2 chip, and it is possible to use a single chip as a light source for an LED lamp.
4, the new structure of the chip
LED core technology, as well as new technology for LED chip structure. The chip structure design mainly considers how to improve the external quantum efficiency, that is, the light extraction efficiency of the chip, improve the heat dissipation performance of the chip, and adopt a new structure and new process in reducing the cost. The chip has many new structures.
(1) Hexahedron light-emitting chip
The hexahedral light-emitting chip refers to all the light on the six sides of the chip, and adopts multi-face surface roughening technology to reduce the reflection of the photon on the interface and improve the light extraction rate.
(2) DA chip structure
Cree uses the advantages of SiC substrate. The DA series has adopted the SiC transparent substrate as the light-emitting surface, and the 3D structure is fabricated on the SiC substrate, that is, the V-shaped groove is provided on the outer side of the SiC substrate, from the V-shaped groove. One side of the light to enhance the light extraction effect of the high refractive index SiC substrate, and is a high current flip chip, the side of the light emitting layer is bonded to the package to obtain high quality heat dissipation, using eutectic soldering, no gold wire, area Almost half of the original, significantly reducing costs and achieving double price/performance. And on the third generation of silicon carbide technology SC3 platform, using the latest matching packaging technology, announced that the luminous efficiency reached 276lm / w.
(3) Single chip white light technology
Samsung uses nano-scale hexagonal pyramid structure technology to make white LEDs, which can grow GaN on semi-polar and non-polar substrates, which is beneficial to the improvement of light extraction. Because the nano structure is small, it can effectively reduce strain and achieve better. Crystal quality, and good heat dissipation. At the same time, it emits green light, yellow light and red light, and its internal quantum efficiency is 61%, 45%, and 29%, respectively. The realization of single-chip multi-color light combined white LED has made breakthrough progress. It will improve the light color quality and avoid the loss of light energy caused by wavelength transfer, and can reduce the packaging process, improve the package reliability and reduce the packaging cost, and become another technical route to realize white LED.
Summary: There are new structures in the development of LED chip structure, and there are continuous breakthroughs in improving light efficiency, heat dissipation performance and cost reduction. More attention should be paid to the development of single-chip multi-color light combined into white LED, which will be another feasible technical route in the development of LED lighting technology.
5, substrate, extension new technology
The following describes several new technologies in the research of LED substrates and epitaxial core technologies that are groundbreaking.
(1) Epitaxial migration growth technique
The University of California uses masking and layered migration techniques to grow low dislocation GaN as shown.
In the schematic, SiO2 is 200 nm thick and SiNX is 120 nm thick. First, a 25 nm nucleation layer is grown at a low temperature of 530 ° C, and then GaN is epitaxially grown at 1040 ° C to perform offset growth, and dislocation growth is obtained, and a dislocation density of 7×10 5 /cm 2 can be obtained, which can greatly improve internal quantum efficiency. Reduce the Droop effect. Adopting innovative technologies on the extension and making breakthroughs will greatly improve LED performance indicators.
(2) 3D silicon-based GaN technology
Aledia announced the cost of manufacturing 3D silicon-based LED chips using 3D silicon-based GaN microwire technology, which is only one-fifth the cost of traditional 2D planar LEDs. The technology is based on an upgraded microwire production process with a large-scale wafer and low-cost material solution that has been used at LETI-CEA in France.
(3) Gallium oxide β-Ga2O3 substrate
Gallium oxide Ga2O3 has various structural forms: α, β, γ, δ, ε, etc. Among them, the β structure is the most stable, and the forbidden band width is 4.8~4.9ev. High-quality, low-defect density Ga2O3 MOSFET has been fabricated, which has excellent devices. potential.
Japan's Tamura Manufacturing Co., Ltd. and its subsidiary, Guangbo Company, used GaN blue light plus phosphors on a β-Ga2O3 substrate. The chip size is 2mm square, plus 6A current, which can obtain 500lm luminous flux, and the planned target is 2000~3000lm.
(4) Rare earth oxide REO composite substrate
According to "Semiconductor Compounds" reported in July 2013: REO composite substrates are grown on Si substrates, and large-area GaN wafers are grown, which have stress relief, reduced warpage, large-area growth, stable REO performance, and reduced cost. And has a higher DBR reflection effect. A composite substrate such as ruthenium oxide or ruthenium oxide has been grown, and good results have been achieved.
(5) Dielectric composite substrate
Shanghai Blu-ray has recently released: Through the combination of buffer layer and dielectric substrate, the parameters have reached or exceeded the level of sapphire PSS substrate, and breakthrough results have been achieved.
Summary: The above-mentioned introduction of several new technology research results is a pioneering innovation. Once industrialized, it will be a subversive technological breakthrough, opening up another important technical route for the development of LED lighting technology.
Abstract In recent years, LED technology has developed rapidly, and breakthroughs in substrate, epitaxy and chip core technologies have been achieved. 1. Graphical Substrate LED Epitaxy Currently, a patterned substrate (PSS) is commonly used. PSS is currently divided into micro-scale PSS and nano-scale nPSS...
In recent years, LED technology has developed rapidly, making breakthroughs in substrate, epitaxy and chip core technologies.